The present invention relates to chromatic dispersion and dispersion slope compensation, and more particularly to chromatic and dispersion slope compensation accumulated in a wavelength division multiplexed optical fiber transmission line.
Fiber optic networks are becoming increasingly popular for data transmission because of their high speed and high capacity capabilities. Wavelength division multiplexing (WDM) is used in such fiber optic communication systems to transfer a relatively large amount of data at a high speed. In wavelength division multiplexing, multiple information-carrying signals, each signal comprising light of a specific restricted wavelength range, may be transmitted along the same optical fiber.
In this specification, these individual information-carrying lights are referred to as either xe2x80x9csignalsxe2x80x9d or xe2x80x9cchannelsxe2x80x9d. The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a xe2x80x9ccomposite optical signalxe2x80x9d.
One common and well-known problem in the transmission of optical signals is chromatic dispersion of the optical signal. Chromatic dispersion refers to the effect wherein the channels comprising an optical signal travel through an optic fiber at different speeds, e.g., longer wavelengths travel faster than shorter wavelengths. This is a particular problem that becomes more acute for data transmission speeds higher than 2.5 gigabytes per second. The resulting pulses of the signal will be stretched, will possibly overlap, and will cause increased difficulty for optical receivers to distinguish where one pulse begins and another ends. This effect seriously compromises the integrity of the signal. Therefore, for a fiber optic communication system to provide a high transmission capacity, the system must compensate for chromatic dispersion. The exact value of the chromatic dispersion produced in a channel of a wavelength-division multiplexed fiber optic communications system depends upon several factors, including the type of fiber and the wavelength of the channel. The common approach is to allow chromatic dispersion to accumulate within spans of fiber and to compensate for dispersion at the ends of spans through the use of in-line dispersion compensators.
FIG. 1 is a graph illustrating the chromatic dispersion characteristics of some conventional optical fibers. The graphs in FIG. 1 represent the Group Velocity Dispersion, D, against wavelength for these conventional optical fibers. The quantity D (ps-kmxe2x88x921-nmxe2x88x921) is defined by the relationship of the following equation:                     D        =                                            ⅆ                              ⅆ                λ                                      ⁢                          (                              1                                  v                  g                                            )                                =                                    1              L                        ⁢                          (                                                ⅆ                                      τ                    g                                                                    ⅆ                  λ                                            )                                                          (                  Eq          .                      xe2x80x83                    ⁢          1                )            
in which xcex is the channel wavelength (nm), xcexdg is the group velocity (km/ps), xcfx84g is the group delay time (ps), and L is the fiber length (km). If xcexdg decreases with increasing wavelength (i.e., longer or xe2x80x9credxe2x80x9d wavelengths travel slower than relatively shorter or xe2x80x9cbluexe2x80x9d wavelengths) then D is positive, otherwise D is negative. Because all three fiber types illustrated in FIG. 1 are deployed in telecommunications systems, the requirements for dispersion compensators vary widely. The chromatic dispersion slope (dispersion slope), S, is given by the first derivatives of the curves shown in FIG. 1. Because dispersion slope is generally not equal to zero, a constant level of dispersion compensation does not accurately negate the dispersion of all channels. This inaccuracy can become a significant problem for high-speed data propagation, long span distances, and/or wide distances between the shortest and longest wavelength channels.
Conventional dispersion compensators include dispersion compensation fiber, chirped fiber Bragg gratings coupled to optical circulators, and conventional diffraction gratings disposed as sequential pairs. A chirped fiber Bragg grating is a special fiber with spatially modulated refractive index that is designed so that longer (shorter) wavelength components are reflected at a farther distance along the chirped fiber Bragg grating than are the shorter (longer) wavelength components. A chirped fiber Bragg grating of this sort is generally coupled to a fiber communications system through an optical circulator. By causing certain wavelength components to travel longer distances than other wavelength components, a controlled delay is added to those components and opposite dispersion can be added to a pulse. However, a chirped fiber Bragg grating has a very narrow bandwidth for reflecting pulses, and therefore cannot provide a wavelength band sufficient to compensate for light including many wavelengths, such as a wavelength division multiplexed light. A number of chirped fiber Bragg gratings may be cascaded for wavelength multiplexed signals, but this results in an expensive system.
A conventional diffraction grating has the property of outputting different wavelengths at different angles. By using a pair of gratings in a coupled spatial arrangement, this property can be used to compensate chromatic dispersion in a fiber communications system. In such a spatial grating pair arrangement, lights of different wavelengths are diffracted from a first grating at different angles. These lights are then input to a second grating that diffracts them a second time so as to set their pathways parallel to one another. Because the different lights travel with different angles between the two gratings, certain wavelength components are made to travel longer distances than other wavelength components. Chromatic dispersion is produced in the spatial grating pair arrangement because the wavelength components that travel the longer distances incur time delays relative to those that travel the shorter distances. This grating-produced chromatic dispersion can be made to be opposite to that of the fiber communications system, thereby compensating the chromatic dispersion within the system. However, a practical spatial grating pair arrangement cannot provide a large enough dispersion to compensate for the relatively large amount of chromatic dispersion occurring in a fiber optic communication system. More specifically, the angular dispersion produced by a diffraction grating is usually extremely small, and is typically approximately 0.05 degrees/nm. Therefore, to compensate for chromatic dispersion occurring in a fiber optic communication system, the two gratings of a spatial grating pair would have to be separated by a very large distance, thereby making such a spatial grating pair arrangement impractical.
Accordingly, there exists a need for an improved tunable chromatic dispersion compensator and dispersion slope compensator which provides simple control of or adjustment of the degree of chromatic dispersion and dispersion slope compensation in a single integrated apparatus. The present invention addresses such a need.
The present invention provides a method and apparatus for tunable chromatic dispersion and dispersion slope compensation for a composite optical signal in an optical fiber transmission system. The composite optical signal includes a plurality of channels, each of the plurality of channels includes a band of wavelengths, where the bands of wavelengths have unwanted dispersion and dispersion slope. An apparatus in accordance with the present invention comprises a collimator optically coupled to an optical signal input and output device, a line focusing lens optically coupled to the collimator at a side opposite to the signal input and output, a Virtually Imaged Phased Array (VIPA) optically coupled to the line focusing lens at a side opposite to the collimator, a transmission diffraction grating with rotational capability optically coupled to the VIPA at a side opposite to the collimator, a lens optically coupled to the transmission diffraction grating at a side opposite to the VIPA and a three-dimensionally curved mirror with translational capabilities optically coupled to the lens at a side opposite to the transmission diffraction grating. A compensator in accordance with the present invention provides simultaneous tunable compensation of dispersion and dispersion slope through rotation of the transmission diffraction grating and/or translation of the three-dimensionally curved mirror.